Near-infrared reflectance type measuring instruments have found wide commercial acceptance for the measurement of organic properties for many foods and other types of materials. Reflectance type NIR instruments require a stable optical standard. To date, most of such instruments have used a standard of ceramic material such as disclosed in U.S. Pat. No. 4,047,032 to Judge et. al., assigned to Technicon Instruments Corp. The standard of this patent has a number of favorable features and advantages including good stability and high reflectivity. However, it does not have the optimum amount of reflectance nor is it consistent with other work performed at the National Bureau of Standards (NBS) to standardize such instruments. There is a need in the art for a standard for spectral reflectance which has an optimum amount of reflectance and is consistent with NBS leadership in the art.
U.S. Pat. Nos. 3,764,364 and 3,956,201, to Jerome A. Seiner and assigned to PPG Industries, Inc., disclose the use of pressed polytetrafluoroethylene (PTFE) powder for providing stable optical standards between 2,400 to 8,000 angstroms wave length range. These types of optical standards have been investigated by the National Bureau of Standards and the results published in the Journal of the Optical Society of America, Volume 71, pages 856-861, July 1981, titled "Reflection Properties of Pressed Polytetrafluoroethylene Powder" by Victor R. Weidner and Jack J. Hsia. The Seiner patents and this technical paper describe the use of PTFE powder as a standard material for reflectance measurements. However, the use of pressed PTFE powder has a significant limitation in that it is fragile and cannot handle normal conditions required in field use of instruments in which such standards would be used.
There is a need in the art to have a standard which can tolerate some of the normal handling conditions which occur in field use of instruments and still not degrade its optical characteristics.